Occurrence of a brass texture and elastic anisotropy in laser blown powder processed superalloy IN718

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Abstract

Challenges are encountered with the laser-blown-powder directed-energy-deposition (LBP-DED) of high-performance alloys such as IN718. Complications that arise include the formation of undesired phases, irregular microstructure, and pronounced texture leading to anisotropic elastic properties. To mitigate these issues, deposition parameters and post-processing treatments must be carefully selected to achieve the desired properties in the final product. In this study it is shown that a Brass texture component ({110} <211>) can be achieved in LBP-DED IN718. EBSD analyses indicate that this texture is enhanced through partial recrystallisation following selected heat treatments, increasing the degree of microstructural anisotropy. Interestingly, resonant ultrasound spectroscopy (RUS) results indicate a decrease in the elastic anisotropy with recrystallisation. This is hypothesised to be a result of the pronounced Brass texture leading to pseudo-isotropic elastic properties along the cubic axes. Further insight into the kinetics of this process have been obtained with in-situ high temperature RUS measurements.

Introduction

Additive manufacturing (AM) methods are highly attractive as these techniques allow near-net-shape component fabrication, increased design freedom and the ability to reduce component weight through elimination of joiners and fasteners [1,2]. Among AM methods, directed-energy deposition (DED) techniques operate on a similar principle to other laser AM systems, with a computer-controlled energy source melting a metallic precursor to fabricate a component layer by layer directly from a 3D computer-aided design (CAD) model [2,3]. The geometrical versatility of some DED techniques, such as laser-blown-powder directed-energy-deposition (LBP-DED), allows for the fabrication and repair of geometrically complex components. Most LBP-DED systems are comprised of a combined coaxial laser and powder jet delivery system. This configuration is highly efficient and allows for omnidirectional fabrication [3]. The option of component repair is particularly attractive for parts with a high unit cost such as the bladed disk assemblies (blisks) that are used in gas turbine applications [4]. These components may suffer damage to the leading edge of the blade aerofoils in service, resulting in a possible reduction in engine performance. The repair of damaged aerofoils therefore offers the potential of substantial cost savings by mitigating the expense of whole component replacement. It has been shown that DED repair methods have the potential to be superior to conventional welding-based repair techniques as lower heat inputs allow reductions in the size of the heat-affected zone (HAZ) and substrate distortions to be achieved [3,6]. These methods also offer the potential benefit of minimising microstructural changes to the substrate and hence loss in mechanical strength. To capitalise on these opportunities, these methods are now being evaluated for the repair of a range of Ni-based superalloy turbine blades [3,4,6,7].

Nickel-based superalloys are widely used in the hottest sections of gas turbine engines because of their resistance to environmental degradation and their exceptional mechanical properties at high temperature [8,9]. Their desirable mechanical properties arise through several strengthening mechanisms [8]. The principal mechanism for most Ni-based superalloys is precipitate strengthening within the disordered face centred cubic (FCC) γ matrix through the formation of an ordered intermetallic phase, for example, the cubic γ′-phase (Ni3Al, L12) or the metastable tetragonal γ″-phase (Ni3Nb, D022) [8,10]. Of the many commercially available superalloys, IN718 is one of the most widely used in industrial high temperature applications due to its high mechanical strength and favourable processability. However, IN718 is limited to service temperatures below 650 °C as prolonged exposure to higher temperatures causes coarsening of the metastable γ″-phase and eventual transformation into the potentially deleterious δ-phase (Ni3Nb, D0a) [5,11,12]. In addition, carbide phases and the Laves topologically close phase (TCP) are commonly encountered within the microstructure of IN718. While carbides can be beneficial to the mechanical properties, TCP phases are generally considered to be detrimental [13].

A particular challenge associated with the LBP-DED technique is the selection of appropriate processing steps to enhance the properties of the deposited material. Porosity, irregular grain size, the precipitation of deleterious phases, and microstructural anisotropy [[1], [14], [16]] often occur and may be partially or fully remedied by both optimisation of the deposition process and utilisation of suitable post-processing of deposited material [10,17,18]. The observed texture of the as-deposited microstructure is reminiscent of directionally solidified material, though there have been difficulties in predicting the mechanical behaviour. Specifically, the observed anisotropy is not limited to the phase constituents, but to defects as well, and this has led to wide distributions in some mechanical testing results due to an orientation dependence [6,7]. Careful control and characterisation of the anisotropy of these materials is therefore essential to ensure component integrity [10,15]. However, certain post-processing operations may be inappropriate for LBP-DED repaired components if the properties of the substrate will be compromised [17,19]. For example, processes such as hot isostatic pressing (HIPing) and high-temperature solution heat treatments may be unsuitable, as they may result in distortion of the part or gross microstructural changes in the substrate.

Such difficulties have been encountered with the repair of IN718 blades in gas turbine engines, where repaired material exhibits a deficit in mechanical properties when compared to the parent material [6,19,20]. The deposited IN718 is typically only subjected to a standard two-stage precipitation heat treatment following deposition and thus, the repaired material retains undesirable features including deleterious phases, residual anisotropy, and an irregular grain size [3]. Reduction of the texture is a particular priority as highly anisotropic properties can lead to design challenges and component instabilities when in service [[21], [22]]. It has been shown that anisotropy in IN718 can be reduced and even eliminated through intensive heat treatments that induce recrystallisation of the deposited material [7,10,18]. However, such heat treatments can compromise the substrate material, and new methods are therefore required to optimise the microstructure of the deposited material whilst minimising any deleterious effects to the substrate [1,21]. The importance of correct post-processing on the properties of Ni-based alloys including IN718 has been well demonstrated by the work of Ozgun et al. [13,23], which highlighted the role of carbides and TCPs in the microstructure of injection moulded IN718 following different heat-treatment protocols.

In the present study, the occurrence of a previously unidentified Brass textural component in LBP-DED IN718 samples is evaluated. The material has been characterised in the as-DED state and following heat treatments in the vicinity of the recrystallisation temperature. This was done to assess the extent to which post-processing may control the extent of this textual component. Microstructural characterisation was performed using scanning electron microscopy (SEM) and chemical and phase analyses were completed using SEM-EDX and electron back-scattered diffraction (EBSD). The evolution of bulk elastic anisotropy was also determined using resonant ultrasound spectroscopy (RUS), with both ex-situ measurements at room temperature and in situ measurements at elevated temperatures to directly study kinetic effects. These results were also correlated to the local textures obtained via EBSD.

Section snippets

Experimental methods

Samples were produced by LBP-DED using IN718 powder of the nominal composition given in Table 1. The IN718 powder was prepared via gas atomisation, yielding a powder with a log-normal particle size distribution of between 40 and 150 μm. Subsequent LBP-DED was carried out using a commercial set-up in line with current industry practices. A bilinear raster pattern was used with an overlap of half the laser diameter and a specific energy of 40 J mm-1. The deposition parameters utilised were

Compositional and microstructural analysis

The measured compositions of the IN718 samples are given in Table 1. SEM-EDX analysis and minority element testing were used to confirm the quoted compositions and identify any deviations from the nominal compositions. Notably, the concentrations of the minority elements in the as-DED samples were higher than quoted in the nominal powder composition. This may be attributed to impurity pick up during deposition giving rise to higher-than-expected concentrations of C and O in the deposited alloy.

Discussion

The steep thermal cycles generated during LBP-DED are expected to influence grain growth, leading to a degree of texture orientated parallel to the direction of local heat flow. As with other AM methods, this is normally the building direction [10]. The fastest crystal growth direction for FCC Ni-based alloys is along <001> directions. As such, for deposition on a polycrystalline substrate, preferential growth typically occurs in grains which have a <001> crystallographic direction parallel to

Conclusions

This study has characterised the microstructure and elastic properties of LBP-DED IN718. Using EBSD mapping it was possible to study the texture under different post-processing conditions. These data were supplemented by RUS which allowed for the characterisation of the elastic properties. It was observed that the elastic anisotropy did not fully correlate with the microstructural anisotropy. Key observations and conclusions in this study are:

A Brass component has been identified as a new

CRediT authorship contribution statement

J.F.S. Markanday: Conceptualization, Validation, Formal analysis, Investigation, Writing – original draft, Visualization. M.A. Carpenter: Methodology, Formal analysis, Resources, Investigation. N.G. Jones: Formal analysis, Writing – review & editing. R.P. Thompson: Formal analysis, Writing – review & editing. S.E. Rhodes: Investigation. C.P. Heason: Resources, Supervision, Funding acquisition. H.J. Stone: Conceptualization, Writing – review & editing, Supervision, Funding acquisition.

Declaration of competing interest

The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: This work was funded by the EPSRC (through an iCase studentship ref: 17000023) and by Rolls-Royce plc.

Acknowledgements

The authors acknowledge funding from the Engineering and Physical Sciences Research Council, UK and from Rolls-Royce plc.

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